1. Field of the Invention
The present invention relates to a fuel cell controller and control method for controlling a fuel cell during a period of purging reaction gas from an anode of the fuel cell, in particular, during startup period thereof.
2. Description of the Related Art
Some fuel cells to be mounted on a fuel cell vehicle, or the like, acquire electric power by inducing chemical reaction of reaction gases. An example of a fuel cell of this type is configured to have an anode and a cathode provided on opposite sides of a solid polymer electrolyte membrane; to supply a fuel gas (for instance, a hydrogen gas) to the anode; and to supply an oxidant gas (for instance, oxygen-containing air) to the cathode, thereby directly extracting chemical energy derived from an oxidation-reduction reaction between these reaction gases as electric energy.
In the fuel cell, water is produced at the cathode side in conjunction with generation of electric power. Some of the thus-generated water is also immersed into the anode by penetrating through the solid polymer electrolyte membrane. In addition, although it is very small quantity, nitrogen contained in the air supplied to the cathode reaches the anode by permeating through the solid polymer electrolyte membrane, and is mixed in the hydrogen gas. Therefore, these impurities in the anode, such as the water content or nitrogen, which ends up with the generation of the fuel cell being instable.
In particular, in a circulatory fuel cell system in which unreacted fuel gas emitted from a fuel cell is recycled and resupplied to the fuel cell for increasing a fuel utilization ratio, an impurity concentration of the anode is likely to increase when stopping condition of the fuel cell lasts for a long time.
For this reason, a technique is proposed in connection with a fuel cell of this type such as disclosed in JP-A-11-97047, where the reaction gases are supplied upon startup with discharging the fuel gas in the fuel cell by opening an emission valve disposed downstream of the anode of the fuel cell, and when an output voltage of the fuel cell attains a predetermined value or higher, the emission valve is closed, whereupon external output is started.
Meanwhile, there are some cases where, when the reaction gas from the anode of a fuel cell has been purged; in particular, immediately after startup, external output power is already requested in accordance with a request for activating another device (for instance, in the case of a fuel cell vehicle whose drive source is a fuel cell, depression of an accelerator pedal), or the like. However, according to the related art, during the course of cleaning operation performed during startup of the fuel cell, the oxidant gas from the cathode is required for the purpose of diluting the reaction gas of the anode to be a predetermined concentration or lower. As a result, the oxidant gas required for such an external output cannot be provided, and a high external output is restricted. Accordingly, there arises a problem that supply of the external output cannot be started until a fuel gas emission valve is closed, and, even when the external output can be supplied, quantity of external output is significantly restricted, thus impairing startup performance.